The subject matter disclosed herein relates to a method of producing feed-through assembly using ceramic to metal bonding used in devices such as an X-ray tube. The method is more particularly related to metal-ceramic brazing with appropriate braze materials.
Metal-ceramic brazing involves the use of interlayer materials which either melt at the bonding temperature, such as a braze, or which involve special coating processes for the material surfaces to be bonded.
Metal-ceramic joints find huge potential applications in engineering structures. Due to the thermal expansion occurring between the metal and ceramic joints, these joints are prone to failures or inconsistent performance. Also, brazing processes introduce a lot of residual stresses due to the effect of immediate cooling from the braze temperature. These residual stresses remain even after the original cause of the stresses leading to breakage in the joints causing failure with prolonged use.
Ceramics are inherently difficult to join either to themselves or metal structures because of their strong ionic and covalent bonding. However, there are several well-established technologies available, including mechanical attachment, adhesives, soldering/brazing and glass-metal sealing. These are often chosen on the basis of temperature requirement of the joint, ease of implementation, functionality etc. There are also other more unusual or application-specific processes, such as microwave bonding, ultrasonic welding and friction welding. Of the many joining processes available, probably the main and most adaptable technique used to join ceramics is brazing.
Ceramic brazing is a liquid phase process that is particularly well suited to preparing joints and seals, and is an established technique for the joining of ceramics. The brazing process involves the joining of two pieces of metal by a fusing layer of brass or spelter between the adjoining surfaces. Here the braze melts and flows in between two materials.
Normally, when a braze alloy or metal is melted between two ceramics a poor joint is likely to form, because of poor wetting. Wetting is a process of obtaining good adhering properties between materials to be brazed. To obtain good wetting properties from the metal surface, the surface of the ceramics or metal is coated with a thin film of metal coating or metal hydride treatment. This helps the braze material to wet or adhere to the surface of the metal/ceramic. Also in a few cases surface treatments like metallization are carried out to obtain good wetting properties.
Filler metal is heated above its melting temperature and distributed between two or more close-fitting parts. The molten metal flows and fills in the gaps by capillary action and is allowed to cool to form a brazed joint. The disadvantage of conventional brazing is the formation of residual stresses developed between the joints during the cooling process. Further the conventional brazing technique has the following drawbacks such as lack of fill, flux entrapment, improper wetting, discontinuous fillet and base metal erosion. In addition, when using conventional brazing, it is difficult to form a leak proof joint between the vacuum chamber and atmospheric chamber.
Hence there exists a need to provide a leak proof joint between the vacuum chamber and the atmospheric chamber of a medical device and producing minimal residual stresses.